Methods for removal of mercury from flue gas

ABSTRACT

Methods and systems for reducing mercury emissions are provided herein. The methods, generally, include the steps of burning a heavy metal containing fuel source and introducing sorbent materials and introducing one or more halogen compounds into the combustion chamber and/or exhaust stream to remove the heavy metal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to U.S. Provisional PatentApplication No. 61/321,696 filed on Apr. 7, 2010 entitled “Methods forRemoval of Mercury from Flue Gas,” the entire contents of which arehereby incorporated by reference.

GOVERNMENT INTERESTS

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PARTIES TO A JOINT RESEARCH AGREEMENT

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INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

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BACKGROUND

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SUMMARY OF THE INVENTION

Various embodiments include a method for reducing heavy metal emissionsincluding the steps of burning a heavy metal containing fuel in acombustion chamber, introducing molecular halogen or one or more halogenprecursors into the combustion chamber or an exhaust stream resultingfrom combustion of the heavy metal containing fuel near the combustionchamber, and injecting activated carbon having a mean particle diameterof less than 15 μm into the exhaust stream. In some embodiments, the oneor more halogen precursors may be calcium hypochlorite, calciumhypobromite, calcium hypoiodite, calcium chloride, calcium bromide,calcium iodide, magnesium chloride, magnesium bromide, magnesium iodide,sodium chloride, sodium bromide, sodium iodide, ammonium chloride,ammonium bromide, ammonium iodide, potassium tri-chloride, potassiumtri-bromide, and potassium tri-iodide. In other embodiments, the halogenprecursors can be calcium bromide, and in still other embodiments, theone or more halogen precursors may be a solid or powder, in an aqueoussolution, or gaseous halogen. In certain embodiments, the molecularhalogen or one or more halogen precursors can be introduced into thecombustion chamber or an exhaust stream resulting from combustion of theheavy metal containing fuel near at a concentration and/or rate ofaddition sufficient to result in a concentration of halogen to produce ahalogen to adsorptive material ratio of at least 0.7 moles of halogenper pound of adsorbent material, or about 0.7 moles/lb to about 5.7moles/lb or about 0.8 moles/lb to about 3.1 moles/lb halogen toadsorbent material. In some embodiments, an aqueous solution of ahalogen precursor having a concentration of about 50% by weight can beintroduced into the combustion chamber or an exhaust stream resultingfrom combustion of the heavy metal containing fuel near the combustionchamber at a rate of less than 10 gallons per hour. In otherembodiments, the halogen precursor may be introduced with the fuelsource, injected into the combustion chamber, injected into the exhauststream near the combustion chamber, or combinations thereof. In someembodiments, the activated carbon has a mean particle diameter of fromabout 2 μm to 10 μm. In other embodiments, the sorbent material can beinjected into the exhaust stream at a rate of less than 5 pounds permillion actual cubic feet (lbs/MMacf), less than about 4 lbs/MMacf, lessthan about 3 lbs/MMacf, or less than about 1 lbs/MMacf based on thetotal exhaust stream flow, and in particular embodiments, the sorbentmaterial may be injected into the exhaust stream at a rate of less than100 lbs/hr. In certain embodiments, the sorbent materials injectedupstream of an air pre-heater (APH), and in particular embodiments,about 90% of the mercury in the fuel source may be removed. In someembodiments, the fuel source can be coal.

DESCRIPTION OF DRAWINGS

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken in connection with the accompanying drawings, inwhich:

FIG. 1 shows a flow chart showing elements of an exemplary coal firedpower plant.

FIG. 2 shows a chart comparing the percent removal of mercury versus theinjection rate for activated carbon.

DETAILED DESCRIPTION

Before the present compositions and methods are described, it is to beunderstood that this invention is not limited to the particularprocesses, compositions, or methodologies described, as these may vary.It is also to be understood that the terminology used in the descriptionis for the purpose of describing the particular versions or embodimentsonly, and is not intended to limit the scope of the present invention,which will be limited only by the appended claims. Unless definedotherwise, all technical and scientific terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art.Although any methods and materials similar or equivalent to thosedescribed herein can be used in the practice or testing of embodimentsof the present invention, the preferred methods, devices, and materialsare now described. All publications mentioned herein are incorporated byreference in their entirety. Nothing herein is to be construed as anadmission that the invention is not entitled to antedate such disclosureby virtue of prior invention.

It must also be noted that as used herein and in the appended claims,the singular forms “a,” “an,” and “the” include plural reference unlessthe context clearly dictates otherwise. Thus, for example, reference to“a combustion chamber” is a reference to “one or more combustionchambers” and equivalents thereof known to those skilled in the art, andso forth.

As used herein, the term “about” means plus or minus 10% of thenumerical value of the number with which it is being used. Therefore,about 50% means in the range of 45%-55%.

As used herein, the term “sorbent material” is meant to encompass allknow materials from any source capable of adsorbing mercury. Forexample, sorbent materials include, but are not limited to, activatedcarbon, natural and synthetic zeolite, silica, silica gel, alumina, anddiatomaceous earths.

Mercury is a known environmental hazard and leads to health problems forboth humans and non-human animal species. Approximately 50 tons per yearare released into the atmosphere in the United States, and a significantfraction of the release comes from emissions from coal burningfacilities such as electric utilities. To safeguard the health of thepublic and to protect the environment, the utility industry iscontinuing to develop, test, and implement systems to reduce the levelof mercury emissions from its plants. In the combustion of carbonaceousmaterials, it is desirable to have a process wherein mercury and otherundesirable compounds are captured and retained after the combustionphase so that they are not released into the atmosphere.

One of the most promising solutions for mercury removal from flue gas isActivated Carbon Injection (ACI). Activated carbon is a highly porous,non-toxic, readily available material that has a high affinity formercury vapor. This technology is already established for use withmunicipal incinerators. Although the ACI technology is effective formercury removal, the short contact time between the activated carbon andthe flue gas stream results in an inefficient use of the full adsorptioncapacity of the activated carbon.

Various embodiments of the invention are directed to methods forremoving heavy metals such as, for example, mercury, from a fluid streamproduced as a result of combustion of a heavy metal containing fuelsource by applying a molecular halogen or halogen precursor to the fuelsource or introducing a molecular halogen or halogen precursor into acombustion chamber during combustion of the fuel source or introducing amolecular halogen or halogen precursor into an exhaust stream resultingfrom the combustion of the fuel source near the combustion chamber andinjecting sorbent material into the exhaust stream, i.e. flue gas,resulting from consumption of the fuel source. In such embodiments, thecombination of applying the molecular halogen or halogen precursor tothe fuel source or injecting the molecular halogen or halogen precursorinto the combustion chamber and injection of sorbent material into theexhaust stream may result in substantial reduction in heavy metalemissions from the exhaust stream while significantly reducing theamount of both the molecular halogen or halogen precursor and thesorbent material used in such methods. In particular embodiments,mercury removal is improved over conventional methods. In someembodiments, greater than about 80% or greater than about 90% of theheavy metal can be removed from the exhaust stream based on the heavymetal content of the fuel source. Thus, the combination achieves similaror improved removal rates while reducing consumption of the molecularhalogen or halogen precursor and sorbent material thereby reducingcosts.

The methods and systems described above may implemented into anyconventional system that involves combustion of a fuel source thatincludes heavy metals. Numerous systems and facilities that burn heavymetal-containing fuels are known and used in the art. For example, someembodiments provide compositions, methods, and systems for reducingemissions of heavy metals from incinerators, including solid wasteincinerators. Other embodiments provide compositions, methods, andsystems for reducing emissions of heavy metals such as mercury thatarise from the combustion of heavy metal containing fossil fuels at, forexample, power plants.

FIG. 1 provides a flow chart depicting relevant portions of an exemplarycoal fired power plant. As indicated in FIG. 1, some such facilities mayinclude a feeding mechanism such as a conveyor 1 for delivering fuelsuch as coal into a furnace or combustion chamber 2 where the fuelsource is burned. The fuel fed into the furnace is burned in thepresence of oxygen with typical flame temperatures in the combustionchamber of the furnace from about 2700° F. to about 3000° F. asindicated to the right of the flow chart. In operation, the fuel may befed into the furnace at a rate suitable to achieve the output desiredfrom the furnace the heat from which can be used to boil water for steamor provide direct heat that can be used to turn turbines that areeventually used to produce electricity (not pictured). From the furnaceor combustion chamber 2, ash, combustion gases, and air move downstream,away from the fireball, into a convective pathway, or exhaust stream,(large arrow to the left of the diagram) that can include various zonesof decreasing temperature as indicated to the right. From the combustionchamber, the heated ash, combustion gases, and air can move through asuperheater 3 and, in cases, a reheater 4 where, for example, water isheated to provide steam which will eventually power a turbine that isused to generate electricity. The ash, combustion gases, and air canalso pass through, for example, an economizer 5 where water fed into thesuperheater 3 and/or reheater 4 is preheated, and an air preheater 6where air that is fed into the combustion chamber 2 is preheated. Thecombustion gases and ash may eventually pass through a baghouse orelectrostatic precipitator 7 where particulate matter is collected. Bythis time, the temperature of the ash, combustion gases, and air isreduced to about 300° F. before being emitted from the stack 8 andreleased into the atmosphere.

In some embodiments, the halogen source may be introduced duringcombustion by injecting molecular halogen or a halogen precursor B intothe combustion chamber 2 or by applying the halogen source directly tothe fuel source prior to combustion A. In other embodiments, the halogenmay be found in the fuel source. For example, waste that includesplastics or rubbers may include halogen containing components that mayrelease halogen ions or molecular halogens during incineration. Invarious embodiments, sorbent material may be injected into the exhauststream anywhere along the convection pathway before emission of the ash,combustion gases, and air into the atmosphere, and in particularembodiments, sorbent material may be injected upstream of the baghouseor electrostatic precipitator 7. In certain embodiments, sorbentmaterial may be injected upstream C of the air preheater (APH) 6, and insome embodiments, sorbent material may be injected into the exhauststream downstream D of the APH 6. In still other embodiments, sorbentmaterial may be injected both upstream C of the APH 6 and downstream Dof the APH 6.

The molecular halogen or halogen precursor of various embodiments may beobtained from any source. For example, in some embodiments, molecularsources such as chlorine gas, bromine gas, or iodine gas can be injectedinto the exhaust stream near the combustion chamber alone or incombination with halogen precursor. In other embodiments, one or morehalogen precursors may be applied to the fuel source, introduced intothe combustion chamber, injected into the exhaust stream near thecombustion chamber, or a combination thereof.

Numerous halogen precursors (halogen precursors) are known in the artand may be used in embodiments of the invention. In some embodiments,the halogen precursor may be a gaseous precursor such as, for example,hydrogen chloride, hydrogen bromide, or molecular chloride or bromide.The halogen precursor may be an organic or inorganic halogen-containingcompound. For example, in some embodiments, the halogen precursor may beone or more inorganic halogen salts, which for bromine may includebromides, bromates, and hypobromites, for iodine may include iodides,iodates, and hypoiodites, and for chlorine may be chlorides, chlorates,and hypochloriates. In certain embodiments, the inorganic halogen saltmay be an alkali metal or an alkaline earth element containing halogensalt where the inorganic halogen salt is associated with an alkali metalsuch as lithium, sodium, and potassium or alkaline earth metal such asberyllium, magnesium, and calcium counterion. Non-limiting examples ofinorganic halogen salts including alkali metal and alkali earth metalcounterions include calcium hypochlorite, calcium hypobromite, calciumhypoiodite, calcium chloride, calcium bromide, calcium iodide, magnesiumchloride, magnesium bromide, magnesium iodide, sodium chloride, sodiumbromide, sodium iodide, ammonium chloride, ammonium bromide, ammoniumiodide, potassium tri-chloride, potassium tri-bromide, potassiumtri-iodide, and the like. In other embodiments, the halogen may from anorganic source, which contains a suitably high level of the halogen.Organic halogen precursors include, for example, methylene chloride,methylene bromide, methylene iodide, ethyl chloride, ethyl bromide,ethyl iodide, chloroform, bromoform, iodoform, carbonate tetrachloride,carbonate tetrabromide, carbonate tetraiodide, and the like.

In some embodiments, the halogen precursor may include one or moreadditional elements such as, for example, a calcium source, a magnesiumsource, a nitrate source, a nitrite source, or a combination thereof.Exemplary calcium and magnesium sources are well known in the art andmay be useful to aid in the removal of sulfur in the flue gas that isreleased from the fuel source during combustion. In such embodiments,the calcium or magnesium source may include inorganic calcium such as,for example, calcium oxides, calcium hydroxides, calcium carbonate,calcium bicarbonate, calcium sulfate, calcium bisulfate, calciumnitrate, calcium nitrite, calcium acetate, calcium citrate, calciumphosphate, calcium hydrogen phosphate, and calcium minerals such asapatite and the like, or organic calcium compounds such as, for example,calcium salts of carboxylic acids or calcium alkoxylates or inorganicmagnesium such as, for example, magnesium oxides, magnesium hydroxides,magnesium carbonate, magnesium bicarbonate, magnesium sulfate, magnesiumbisulfate, magnesium nitrate, magnesium nitrite, magnesium acetate,magnesium citrate, magnesium phosphate, magnesium hydrogen phosphate,and magnesium minerals and the like, or organic magnesium compounds suchas, for example, magnesium salts of carboxylic acids or magnesiumalkoxylates. In certain embodiments, the calcium or magnesium source maybe associated with the halide precursor such as, for example, calciumbromide, magnesium bromide, calcium chloride, magnesium chloride,calcium iodide, magnesium iodide, and the like. Nitrate and nitritesources are also well known in the art and any source of nitrate ofnitrite can be formulated with halogen precursor.

The halogen precursor may be a solid such as a powder, a liquid, or agas. For example, in some embodiments, the halogen precursor may be anaqueous solution that can be sprayed onto the fuel source such as coalbefore combustion or can be injected into the combustion chamber orexhaust stream near the combustion chamber. A liquid halogen precursorcomposition may be prepared at any suitable concentration. For example,in some embodiments, an aqueous solution of a halogen precursor such as,for example, calcium bromide or calcium chloride, may have aconcentration of up to about 75%, and in other embodiments, the halogenprecursor concentration in the aqueous solution may be up to about 60%by weight, 55% by weight, 50% by weight, 45% by weight, or 40% by weightor any concentration between these values. In still other embodiments,an aqueous solution of a halogen precursor may include about 10% toabout 75% by weight, about 20% to about 60% by weight, about 30% toabout 55% by weight, or about 40% to about 55% by weight of the halogenprecursor. Similarly, in other embodiments, dry, powdered halogenprecursor may be applied to the coal at a concentration necessary toachieve a similar concentration of halogen in the flue gas stream.

In various embodiments, the molecular halogen or halogen precursor,which may be in solid, such as a powder, liquid, or a gaseous form, maybe continuously supplied to the combustion chamber or providedincrementally during combustion. The rate of addition of the molecularhalogen and halogen precursor may vary among embodiments and may depend,for example, on the rate of combustion of the fuel source, the origin ofthe fuel source, the amount of mercury in the fuel source, theadsorption of mercury, and the like. For example, in some embodiments,an about 40% to about 55% by weight aqueous solution of a halogenprecursor such as, for example, calcium bromide or calcium chloride, maybe introduced into a combustion chamber or injected into an exhauststream near the combustion chamber at a rate of about 500 gallons/hr orless, and in other embodiments, an about 40% to about 55% by weightaqueous solution of the halogen precursor introduced into a combustionchamber or injected into an exhaust stream near the combustion chamberat a rate of about 400 gallons/hr or less, 300 gallons/hr or less, 200gallons/hr or less, or 100 gallons/hr or less. In certain embodiments,an about 40% to about 55% by weight aqueous solution of the halogenprecursor introduced into a combustion chamber or injected into anexhaust stream near the combustion chamber at a rate of less than 50gallons/hr or less than 25 gallons/hr or less than 20 gallons/hr.

The feed rate of the molecular halogen or halogen precursor may varyamong embodiments and may vary depending on, for example, the feed rateof the fuel source and/or the rate of consumption of the fuel source.For example, a combustion chamber burning about 330 tons/hr of a fuelsource such as coal in six mills each burning about 55 tons/hr whereabout 10 gal/hr of a 50% by weight aqueous solution of calcium bromide(CaBr₂) is introduced into the combustion chamber during burning canresult in about 125 μmm bromine added to the coal based on dry weight.Thus, in various embodiments, the concentration and/or feed rate themolecular halogen or halogen precursor may be modified based on the rateof consumption of the fuel source such that up to about 400 μmm (drybasis), up to about 500 μmm (dry basis) or up to about 700 μmm (drybasis) bromine may be added the fuel source. In some embodiments, about50 μmm to about 500 μmm (dry basis), about 75 μmm to about 400 μmm (drybasis), about 100 μmm to about 300 μmm (dry basis), or about 125 μmm toabout 200 μmm (dry basis) of bromine may be added to the fuel source.

In some embodiments, the methods and systems described herein may beutilized in a multi-stage furnace having for example, a primary andsecondary combustion chambers, a rotary kiln, afterburning chambers, andany combinations thereof. In such embodiments, molecular halogen orhalogen precursor in a solid or liquid form may be introduced into anyone or any combination of the chambers of the furnace. For example, insome embodiments, the molecular halogen or halogen precursor may beintroduced into one combustion chamber, and in other embodiments, themolecular halogen or halogen precursor may be introduced into acombination of combustion chambers. In still other embodiments,molecular halogen or halogen precursor may be introduced into one ormore combustion chambers and into an exhaust stream after combustion.

In certain embodiments, the halogen precursor may be introduced into oneor more combustion chambers and/or exhaust stream as an aqueous solutionthat is sprayed or injected into the chamber or exhaust stream. Forexample, in some embodiments, an aqueous solution of a halogen precursormay be sprayed or injected into a combustion gas stream downstream of awaste-heat boiler. In still other embodiments, an aqueous solution ofthe halogen precursor may be introduced into a recirculated substreamsuch as, for example, a recirculated flue gas, recirculated ash, orrecirculated fly ash. While embodiments are not limited by the zonewhere the molecular halogen or halogen precursor is introduced into theexhaust gas stream, the temperature in the injection zone should besufficiently high to allow dissociation and/or oxidation of theelemental halogen from the halogen precursor. For example, thetemperature at the injection zone may be greater than about 1000° F.,and in some embodiments, greater than about 1500° F.

Without wishing to be bound by theory, halogens from the molecularhalogen or halogen precursor can oxidize with heavy metals released fromthe fuel source when it is burned in the combustion chamber. In general,oxidized heavy metals, such as mercuric halide species are adsorbable byalkaline solids in the exhaust stream such as fly ash, alkali fusedacidic ash (e.g., bituminous ash), dry flue gas desulfurization solidssuch as calcium oxide, calcium hydroxide or calcium carbonate, andremoved from the flue gas by commonly used heavy metal control systemssuch as, for example, electrostatic precipitators, wet flue gasdesulphurization systems, fabric filters, and baghouses. In certainembodiments, oxidized heavy metals may be adsorbed by activated carbon.Without wishing to be bound by theory, the rate at which a solution of ahalogen precursor may be significantly reduced by combining theapplication of a halogen-containing composition with injection ofsorbent material into the fluid stream of the combustion gases even whenthe mercury content of the fuel source is relatively high.

Activated carbon may be used in any embodiment. In such embodiments, theactivated carbon may be obtained from any source and can be made from avariety of starting materials. For example, suitable materials forproduction of activated carbon include, but are not limited to, coals ofvarious ranks such as anthracite, semianthracite, bituminous,subbituminous, brown coals, or lignites; nutshells, such as coconutshell; wood; vegetables such as rice hull or straw; residues orby-products from petroleum processing; and natural or syntheticpolymeric materials. The carbonaceous material may be processed intocarbon adsorbents by any conventional thermal or chemical method knownin the art. The adsorbents will inherently impart different surfaceareas and pore volumes. Generally, for example, lignites can result incarbon having surface areas about 500-600 m²/g and, typical fiber-basedcarbons areas are about 1200-1400 m²/g. Certain wood-based carbons mayhave areas in the range of about 200 m²/g, but tend to have a very largepore volume.

Surface area and pore volume of coal based carbon may also be made toallow for some control of surface area and pore volumes and pore sizedistributions. In some embodiments, the activated carbon adsorbent mayhave large surface area as measured by the Brunauer-Emmett-Teller(“BET”) method, and may have a substantial micropore volume. As usedherein, “micropore volume” is the total volume of pores having diameterless than about 2 nm. In some embodiments, suitable carbon adsorbentsmay have a BET surface areas greater than about 10 m²/g or about 50m²/g, greater than about 200 m²/g, or greater than about 400 m²/g. Inother embodiments, the carbon adsorbent may have a micropore volume ofgreater than about 5 cm³/100 g, and in still other embodiments, theadsorbent may have a micropore volume greater than about 20 cm³/100 g.

Sorbent materials, such as activated carbon, of various sizes have beenused to capture heavy metals in systems currently utilized, and any sizesorbent material can be used in various embodiments. For example, insome embodiments, the sorbent material may have a mean particle diameter(MPD) of about 0.1 μm to about 100 μm, and in other embodiments, the MPDmay be about 1 μm to about 30 μm. In still other embodiments, the MPD ofthe sorbent material may be less than about 15 μm, and in someparticular embodiments, the MPD may be about 2 μm to about 10 μm, about4 μm to about 8 μm, or about 5 μm or about 6 μm.

In some embodiments, the sorbent material may be treated with, forexample, a halogen containing salt. For example, in various embodiments,the sorbent material may be impregnated with a bromine by, for example,immersing the sorbent material in a solution of a hydrogen bromide or astream of elemental bromine gas for sufficient time to allow the bromineto impregnate the sorbent material. Various methods for impregnating thesorbent material and types of impregnated sorbent material are known andused in the art, and any such sorbent material may be used inembodiments.

The sorbent material may be injected into the exhaust stream anywherealong the convection pathway downstream of the combustion chamber andbefore the exhaust is emitted from the stack. The sorbent material ofvarious embodiments may generally be injected downstream of a heavymetal control systems such as, for example, electrostatic precipitators,wet flue gas desulphurization systems, fabric filters, and baghouses orother ash or fly ash collection means where particulate matter can becollected and upstream of the combustion chamber. In certainembodiments, the sorbent material may be injected at any zone in theconvection pathway having a temperature of less than about 700° F., lessthan about 500° F., less than about 400° F. or less than about 350° F.For example, in some embodiments, sorbent material may be injected intoan exhaust stream either upstream or downstream of an air pre-heater(APH), and in other embodiments, the sorbent material may be injectedupstream of an air pre-heater (APH).

In some embodiments, the rate of injection of the sorbent material maydepend upon the flow rate of the exhaust stream. For example, in a planthaving a exhaust (flue) gas flow rate of about 2,000,000 actual cubicfeet per minute (acfm) in which about 100 lbs/hr of sorbent material isinjected into exhaust stream in the ductwork of the plant, the rate ofaddition of sorbent material is about 0.8 pounds per million actualcubic feet (lbs/MMacf). Therefore, in various embodiments, the injectionrate of the sorbent material may vary depending up on the flow rate ofthe exhaust gas in the ductwork. In such embodiments, the rate ofaddition of sorbent material based on the flow rate of the exhaust gasmay be up to about 4 lbs/MMacf or up to about 5 lbs/MMacf. In otherembodiments, the rate of addition of the sorbent material based on theflow rate of the exhaust gas may be from about 0.25 lbs/MMacf to about 5lbs/MMacf, about 0.5 lbs/MMacf to about 4.0 lbs/MMacf, or about 0.75lbs/MMacf to about 3.0 lbs/MMacf, and in particular embodiments, therate of addition may be about 0.75 lbs/MMacf to about 1.5 lbs/MMacf.

Particular embodiments, for exemplary purposes, include methods andsystems including the introduction of a halogen precursor, such as,calcium bromide, calcium chloride, sodium bromide, or sodium chloride,into a combustion chamber where a heavy metal containing fuel source isbeing burned, and injection of sorbent material having an MPD of lessthan about 15 μm into an exhaust stream upstream of a heavy metal and/orparticulate control systems such as, for example, electrostaticprecipitators, wet flue gas desulphurization systems, fabric filters,and baghouses or other ash or fly ash collection means where particulatematter can be collected. In some such embodiments, less than about 10gallons/hour of the an aqueous halogen precursor may be introduced intothe combustion chamber, and less than about 100 lbs/hour of sorbentmaterial may be injected into the exhaust stream. As a result of suchtreatment, mercury emission from the plant employing such methods andsystems may be reduced by greater than about 80% and in someembodiments, greater than 90%.

Further embodiments, include methods for reducing mercury emissions fromflue gas in which the ratio of halogen to sorbent material provided isfrom about 0.7 to about 4.6 moles of halogen per pound of activatedcarbon, and in some embodiments, from about 0.8 to about 3.1 or about1.2 to about 2.0 moles of halogen per pound of activated carbon. In suchembodiments, the sorbent material may have an MPD of less than about 15μm and, in certain embodiments, the sorbent material may have an MPD ofless than about 10 μm. In still other embodiments, the sorbent materialmay have an MPD of about 6 μm or less. The halogen and sorbent materialmay be provided anywhere during the process. For example, in someembodiments, the halogen may be applied to the fuel source beforecombustion, and in other embodiments, the halogen may be introduced intothe combustion chamber while the fuel is burned. In still otherembodiments, the halogen may be introduced into the flue gas streameither before or after the sorbent material. In further embodiments, thehalogen may be provided with the activated carbon. For example, in someembodiments, the halogen may be injected into the flue gas streamseparately with the activated carbon, and in other embodiments, thehalogen may be applied to the sorbent material before it is introducedinto the flue gas stream.

In embodiments in which the halogen is applied to the sorbent materialbefore being injected into the flue gas stream, the ratio of halogen tosorbent material may be the same as the ratio of halogen to sorbentmaterial when sorbent material is introduced separately. For example, insome exemplary embodiments, a halogen salt such as any of the halogensalts described above may be applied to an adsorbent material having anMPD of less than 15 μm, less than 12 μm, less than 10 μm in a ratio offrom about 0.14 to about 1.0 pounds of halogen salt per pound of sorbentmaterial to provide a composition that is from about 12 wt. % to about50 wt. % halogen salt or about 15 wt. % to about 40 wt. % halogen salt.In another exemplary embodiment, a halogen salt such as calcium bromide(CaBr₂) or ammonium bromide (NH₄Br) may be applied to sorbent materialhaving an MPD of about 6 μm at a ratio of about 0.43 pounds of halogensalt per pound of sorbent material or about 30 wt. % halogen salt, andthe sorbent material/halogen salt combination may be introduced into theflue gas stream. These ratios can also be expressed as moles of halogenper pound of adsorbent material. For example, in some embodiments, theratio of moles of halogen per pound of sorbent material may be fromabout 0.7 moles/lb to about 5.7 moles/lb, 0.8 moles/lb to about 3.1moles/pound or any ratio there between, and in particular embodiments,the ratio of halogen per pound of sorbent material can be 2.0 moles/lb.In such embodiments, the halogen salt may be applied by conventionalimpregnation process or the halogen salt may be applied by mixing drysorbent material with dry halogen salt. In other embodiments, thesorbent material can be impregnated using a gaseous halogen. In certainembodiments, such as those described above, the sorbent material may beactivated carbon.

Coal fired power plants utilizing conventional methods for reducingmercury emissions where a halogen precursor is introduced into acombustion chamber and no sorbent material is injected into the exhaustgenerally inject halogen precursor at a rate of greater than 20gallons/hour to reduce the mercury emission sufficiently. Coal firedpower plants that utilize sorbent material injection without introducinga halogen precursor during combustion can inject greater than about 250lbs/hour of sorbent material into the exhaust stream to effectivelyreduce mercury emissions. In contrast, some embodiments of the inventionprovide mercury reduction of greater than about 80% or greater than 90%while using less than about 10 gallons/hour of a halogen precursor andless than 100 lbs/hour of an activated carbon, and in particularembodiments, less than 100 lbs/hour of sorbent material having a MPD ofless than about 15 μm. This is a dramatic and surprising reduction inthe amount of consumables necessary to effectively reduce mercuryemissions to below regulatory levels. Such embodiments, therefore,provide substantial economic advantages over currently used methods forreducing mercury emission, while simultaneously reducing the amount ofash produced by plants that employ sorbent material injection and theamount of halogen precursor consumed.

In some embodiments, mercury levels can be monitored with conventionalanalytical equipment using industry standard detection and determinationmethods, and in such embodiments, monitoring can be conductedperiodically, either manually or automatically. For example, mercuryemissions can be monitored once an hour to ensure compliance withgovernment regulations and to adjust the rate of halogen precursorintroduction into the combustion chamber, the rate of sorbent materialinjection, or both. Mercury can be monitored in the convective stream atsuitable locations. For example, in some embodiments, mercury releasedinto the atmosphere can be monitored and measured on the clean side of aparticulate control system.

EXAMPLES

Although the present invention has been described in considerable detailwith reference to certain preferred embodiments thereof, other versionsare possible. Therefore the spirit and scope of the appended claimsshould not be limited to the description and the preferred versionscontained within this specification. Various aspects of the presentinvention will be illustrated with reference to the followingnon-limiting examples.

Example 1

A coal-fired power plant fitted with a system to add calcium bromideonto the coal prior to the combustion chamber and lances for injectingactivated carbon into the ductwork of the power plant at variouslocations was utilized for testing. Coal burned at this facility wasperiodically tested for mercury content to ensure accuracy of mercuryremoval testing. Various powdered activated carbons (PACs) tested atthis facility are provided in Table 1.

TABLE 1 Powdered Activated Carbon (PAC) Identifier Particle Size (MPD)Brominated Std 16 μm No Std Br 16 μm Yes PAC 6  6 μm No PAC 30 30 μm NoEach of the PACs described in Table 1 was injected into the exhauststream of the plant down stream of the APH at rate of about 100 lbs/hror about 200 lbs/hr either with or without calcium bromide (CaBr₂)injection into the combustion chamber. The results are provided in Table2 and are illustrated in FIG. 2.

TABLE 2 Injection Rate Raw Data Symbol PAC Injection FIG. 2 ParticleCaBr Rate (lbs/hr) Removal (%) ▪ Std. 0 100 48.2 Std. 0 200 60.0 ▴ None1X 0 20.0 Std. 1X 100 67.5 Std. 1X 200 77.6 □ None 2X 0 33.4 Std. 2X 20083.3 Δ None 3X 0 39.2 ● None 4X 0 37.1 Std. 4X 200 88.0 ○ Std. 8X 20087.4 * Std. Br 0 100 70.4 Std. Br 0 200 82.7 Std. Br 0 200 79.4 × PAC 300 100 36.5 PAC 30 0 200 48.4 ⋄ PAC 6  0 100 55.3 PAC 6  0 200 67.6 ♦ PAC6  4X 100 87.4 PAC 6  4X 200 92.7

As indicated in FIG. 1, CaBr₂ alone, PAC injection rate 0, resulted inless than about 50% mercury removal based on the mercury content of thecoal consumed. The addition of PAC at 100 lbs/hr (PAC 30, PAC 16, PAC 6)resulted in similar reduction in mercury emission, about 50%, whichvaried slightly depending on the MPD of the PAC. The combination ofCaBr₂ injection into the combustion chamber and PAC injection in theexhaust stream (Std+1×CaBr₂) showed improved reduction in mercuryemission, as did the injection of brominated PAC (Std Br) into theexhaust stream. Notably, the combination of CaBr₂ injection into thecombustion chamber and injection of a PAC having a smaller MPD into theexhaust stream (PAC 6+4×CaBr₂) resulted in nearly 90% reduction inmercury emissions, which represents almost 20% greater reduction inmercury emissions over brominated PAC (Std Br) and larger MPD PAC andCaBr₂ (Std.+1×CaBr₂). Similarly, when the injection rate for PAC wasincreased to 200 lbs/hr, small MPD PAC outperformed brominated PAC (Std.Br) and larger MPD PAC and various injection rates of CaBr₂ (Std.1×CaBr₂; Std. 2×CaBr₂; Std. 3×CaBr₂; Std. 4×CaBr₂; and Std. 8×CaBr₂).

Example 2

Further testing was carried out to determine the injection rate for agiven aqueous solution of CaBr₂ and PAC when the PAC is injected intothe exhaust stream upstream of the APH (Post APH Injection) anddownstream of the APH (Pre APH Injection) required to obtain 90% removalof mercury from plant emissions. The results are provided in Tables 3and 4, respectively.

TABLE 3 Consumption at 90% mercury removal POST APH INJECTIONIndentifier CaBr₂ (gal/hr) PAC (lbs/hr) #/MMacf Std. 20 300 2.5 PAC 6 20150 1.2 Std. Br 420 3.4

TABLE 4 Consumption at 90% mercury removal PRE APH INJECTION IndentifierCaBr₂ (gal/hr) PAC (lbs/hr) #/MMacf Std. 18 125 1.0 PAC 6 6 60 0.5 Std.Br 320 2.6Tables 3 and 4 show that a rate of CaBr₂ injection of 20 gal/hr and aPAC injection rate of 150 lbs/hr is sufficient to remove 90% of themercury from the coal tested when small MPD PAC (PAC 6) is injected downstream of the APH whereas twice as much large MPD PAC (Std.) is requiredto achieve a similar result. When the PAC is injected upstream of theAPH, 6 gal/hr of CaBr₂ and 60 lbs per hour of small MPD PAC (PAC 6) isnecessary to remove 90% of the flue gas mercury at the same plantwhereas 18 gla/hr of CaBr₂ and 125 lbs/hr of standard MPD PAC (Std.) arerequired to achieve the same result. These data demonstrate that adecrease in carbon particle size, especially below about 12 μm or about10 μm, creates its own synergistic effect in that, surprisingly, bothless carbon and less halogen are needed for the same level of mercuryremoval, especially at levels around or above 90% mercury removal. Thecombined savings in both halogen and sorbent result in greatly improvedeconomics as well as fewer balance-of-plant impacts such as reducedcarbon in the fly ash, allowing more of the ash to retain commercialvalue as a concrete additive.

1. A method for reducing heavy metal emissions comprising: burning aheavy metal containing fuel in a combustion chamber; introducingmolecular halogen or one or more halogen precursors into the combustionchamber or an exhaust stream produced as a result of the burning of theheavy metal containing fuel; and introducing a sorbent having a meanparticle diameter of less than 15 μm into an exhaust stream resultingfrom burning of the heavy metal containing fuel in the combustionchamber; wherein the halogen and sorbent material are introduced at aratio of from about 0.7 to about 5.7 moles of halogen per pound (lb) ofsorbent material.
 2. The method of claim 1, wherein the halogen andsorbent material are introduced at a ratio of from about 0.8 to about3.1 moles of halogen per pound (lb) of sorbent material.
 3. The methodof claim 1, wherein the halogen and sorbent material are introduced at aratio of about 2.0 moles of halogen per pound (lb) of sorbent material.4. The method of claim 1, wherein the sorbent material is activatedcarbon.
 5. The method of claim 1, wherein the one or more halogenprecursors are selected from the group consisting of calciumhypochlorite, calcium hypobromite, calcium hypoiodite, calcium chloride,calcium bromide, calcium iodide, magnesium chloride, magnesium bromide,magnesium iodide, sodium chloride, sodium bromide, sodium iodide,ammonium chloride, ammonium bromide, ammonium iodide, potassiumtri-chloride, potassium tri-bromide, and potassium tri-iodide.
 6. Themethod of claim 1, wherein the halogen precursor is selected from thegroup consisting of calcium bromide (CaBr₂), ammonium bromide (NH₄Br),and combinations thereof.
 7. The method of claim 1, wherein the halogenprecursor are in an aqueous solution.
 8. The method of claim 1, whereinthe halogen precursor is a solid.
 9. The method of claim 1, wherein thehalogen or halogen precursor is a gas.
 10. The method of claim 1,wherein the halogen or halogen precursor is introduced into the exhauststream having a temperature greater than about 1000° F.
 11. The methodof claim 1, wherein the sorbent material has a mean particle diameter(MPD) of from about 2 μm to 10 μm.
 12. The method of claim 1, whereinthe sorbent material is injected into the exhaust stream at a rateselected from the group consisting of less than about 5 pounds permillion actual cubic (lbs/MMacf), less than about 4 lbs/MMacf, less thanabout 3, lbs/MMacf, and less than about 1 lbs/MMacf.
 13. The method ofclaim 1, wherein the sorbent material is injected into the exhauststream at a rate of less than 100 pounds/hr.
 14. The method of claim 1,wherein the sorbent material is injected upstream of an air pre-heater(APH).
 15. The method of claim 1, wherein about 90% of the mercury inthe fuel source is removed.
 16. The method of claim 1, wherein the fuelsource is coal.
 17. The method of claim 1, wherein the halogen orhalogen precursor is introduced into the combustion chamber.
 18. Themethod of claim 1, wherein the halogen precursor is applied to the fuelbefore the fuel is introduced into the combustion chamber.
 19. Themethod of claim 1, wherein the halogen is applied to the sorbentmaterial.
 20. The method of claim 17, wherein the sorbent material has amean particle diameter (MPD) of from about 2 μm to 10 μm.
 21. The methodof claim 17, wherein the halogen is provided to the sorbent material ata concentration of from about 0.7 moles of halogen per pound of sorbentmaterial to about 5.7 moles of halogen per pound of sorbent material.22. The method of claim 17, wherein the halogen is provided to thesorbent material at a concentration of from about 0.8 moles of halogenper pound of sorbent material to about 3.1 moles of halogen per pound ofsorbent material.
 23. The method of claim 17, wherein sorbent materialhas a mean particle diameter (MPD) of about 6 μm and the halogen isapplied to the sorbent material to produce a final concentration ofabout 2.0 moles of halogen per pound of sorbent material.
 24. The methodof claim 17, wherein the sorbent material is activated carbon having aparticle diameter (MPD) of about 6 μm and the halogen is applied to theactivated carbon to produce a final concentration of about 2.0 moles ofhalogen per pound of sorbent material.
 25. The method of claim 17,wherein the halogen is applied to the sorbent material by wetimpregnation.
 26. The method of claim 17, wherein the halogen is appliedto the sorbent material by combining dry activated carbon with dryhalogen salt.
 27. The method of claim 26, wherein the halogen salt isselected from the group consisting of calcium bromide (CaBr₂), ammoniumbromide (NH₄Br) and combinations thereof.
 28. A flue gas sorbentcomprising: a sorbent material having a mean particle diameter (MPD) ofless than about 15 μm; and one or more halogens associated with thesorbent material at a ratio of from about 0.7 to about 5.7 moles ofhalogen per pound (lb) of sorbent material.
 29. The flue gas sorbent ofclaim 28, wherein the one or more halogens are associated with thesorbent material at a ratio of from about 0.8 to about 3.1 moles ofhalogen per pound (lb) of sorbent material.
 30. The flue gas sorbent ofclaim 28, wherein the one or more halogens are associated with thesorbent material at a ratio of about 2.0 moles of halogen per pound (lb)of sorbent material.
 31. The flue gas sorbent of claim 28, wherein thesorbent material has a mean particle diameter (MPD) of from about 2 μmto 10 μm.
 32. The flue gas sorbent of claim 28, wherein sorbent materialhas a mean particle diameter (MPD) of about 6 μm and the halogen isassociated with the sorbent material at a final concentration of about2.0 moles of halogen per pound of sorbent material.
 33. The flue gassorbent of claim 28, wherein the sorbent material is activated carbonhaving a particle diameter (MPD) of about 6 μm and the halogen isassociated with the activated carbon at a final concentration of about2.0 moles of halogen per pound of sorbent material.
 34. The flue gassorbent of claim 28, wherein the halogen is applied to the sorbentmaterial by wet impregnation.
 35. The flue gas sorbent of claim 28,wherein the halogen is applied to the sorbent material by combining dryactivated carbon with dry halogen salt.
 36. The flue gas sorbent ofclaim 35, wherein the halogen salt is selected from the group consistingof calcium bromide (CaBr₂), ammonium bromide (NH₄Br) and combinationsthereof.